Vertical pumps are used extensively in industrial applications in wells and other structures where liquids are contained. Oftentimes the contained liquid is highly toxic, corrosive, or is subject to temperature extremes that make an enclosed containment necessary for the protection of nearby personnel and/or the environment. In such hazardous environments, pumps are not readily accessible for observation, troubleshooting, and/or maintenance. It is therefore highly important that vertical pumps used under such conditions be as reliable as possible, especially in industrial applications where it is important to maintain profitability.
Vertical pumps are often of relatively small diameter in comparison to their length. Vertical pumps can be of any length within the confines of acceptable design practices, and are frequently custom built to fit the specific requirements of a particular containment structure. Some vertical pumps include a mounting plate at or near an upper end which is designed to mate with a containment structure, such that the pump extends downward into the containment structure without any other support. In enclosed containment structures such as those holding hazardous liquids, the opening through which a pump is inserted is often quite small because of a need to minimize any emissions from the containment through the opening.
Vertical pumps which are suspended only from above and which include long, unsupported, sections with a low diameter-to-length ratio are prone to oscillation caused by sources such as rotodynamic imbalance and hydrodynamic imbalance, as well as external sources such as seismic disturbances, waves, and/or currents. Such vertical pumps become increasingly susceptible to oscillation as liquid levels are lowered within a containment structure, because of loss of the natural damping action that liquid submergence provides to a vertical pump.
Most vertical pumps are classified as non-rigid structures, in that the operating frequency of the rotor is higher than the 1st natural frequency of the pump structure. In some applications, the pump structure can be designed so that it does not have any natural oscillation frequencies which conflict with the operating frequency of the rotor. However, use of variable speed controls in conjunction with vertical pumps often creates a situation where the operating speed of the pump can be brought sufficiently close to a natural oscillation frequency of the pump structure so that undesirable resonances are generated.
Vertical pump resonances can often reach amplitudes which are detrimental to the continuous operation of the pump. If the pump is installed within a containment structure, and is therefore not visible to the operators, the remoteness of the problem, coupled with the incompatibility of many liquids with sensing instrumentation, can result in premature equipment failure without forewarning to the operators. Such an unforeseen failure of a vertical pump can cause loss of production, as well as injury to personnel and damage to ancillary equipment.
One approach to suppressing resonances in vertical pumps is to use a Tuned Mass Damper (TMD). TMD's are well known in the field of harmonic control as a means of damping resonant vibrations by combining a relatively small mass with a spring and damping device so as to dissipate the energy created by an oscillating motion of a larger mass.
When a structure begins to oscillate or sway, movement of the structure is transmitted to a TMD, setting it into an out-of-phase motion by means of the spring. Ideally, the frequencies and amplitudes of the TMD and the structure should nearly match, so that whenever the structure is set in motion, the TMD creates an equal and opposite out-of-phase motion, keeping the horizontal displacement of the structure at or near zero.
The effectiveness of a TMD is dependent on the mass ratio of the TMD to the structure itself, the ratio of the frequency of the TMD to the oscillation frequency of the structure, and the damping ratio of the TMD, i.e. how well the damping device dissipates energy.
Heretofore, TMDs have not been applied to vertical pumps due to the difficulty of designing an effective TMD which can be readily fit through the pump installation opening and contained within the confines provided by the containment structure in which the vertical pump is installed.
Nor has there been a provision in vertical pump design for damping without the addition of appreciable mass to the pump. Unlike permanent vertical structures such as buildings and radio towers, a vertical pump must be routinely removed from its containment structure and transitioned between the vertical orientation in which it operates and a horizontal orientation in which maintenance is performed. Longitudinal alignment is critical to the reliability of a vertical pump, which can be easily prone to bending when force is applied normal to the pump's longitudinal axis. The addition of any large mass to the pump prior to installation necessitates either special provision for lifting or special procedures so as not to introduce detrimental bending forces which would disturb the alignment of the pump while it is being transitioned between the horizontal and vertical orientations.
Instead of attempting to incorporate a vibration damping solution, pump manufactures typically just provide instructions to users advising them to restrict the operational range of the vertical pump so as to avoid generating damaging resonances coincident with the natural frequencies of the pump structure. These limitations are inconvenient to users, and in some cases the operating restrictions can limit the productivity of the vertical pump.
What is needed, therefore, is a vibration damping device for a vertical pump which can prevent the vertical pump from being damaged by structural oscillations and resonances originating from causes such as rotodynamic and/or external sources, and which is compatible with exposure to a broad range of chemicals and environmental conditions. It is further desirable that the vibration damping device be compact and integral to the pump structure, so that it fits within the available space and can be readily inserted through pump installation openings and into enclosed containment structures. Furthermore, it is desirable that the vibration damping device not add appreciable mass to the pump during transition of the pump between horizontal and vertical orientations.